1. Field of the Invention
The present invention relates to an ink jet recording apparatus, and specifically, an ink jet recording apparatus provided with a dust removal unit in an ink supply system used for supplying an ink from an ink tank to a recording head.
2. Description of Prior Art
In such an apparatus as represented by an ink jet recording apparatus which has a fluid route with a relatively small cross-section used for transporting fluids in it, it is required to prevent the fluid route from being plugged by dust included in the fluid, substances that come out of the fluid and even bubbles in the fluid.
In the ink jet recording apparatus, if the above-described dust or the like included in an ink fluid go into the neighborhood of the orifice of the recording head, it may occur that the orifice is plugged and that ink droplets fail to be ejected. In order to solve this problem, it is conventional that a filter for removing the fine dust, the bubbles and the like is placed anywhere in the ink supply system having a supply route from the ink tank through a common fluid reservoir and an ink path of the recording head to the orifice.
In an ink jet recording apparatus, as an example of structure for an ejection recovery operation for the recording head, there may be provided the structure in which a fresh ink fluid without bubbles and the like can be supplied to the recording head by causing circulation flow of the ink in the recording head and the ink supply system.
In an ejection recovery operation by using circulation flow of the ink fluid, the ink fluid is pressurized and driven by a pump and transported from the ink tank through a forward ink route to the recording head, and transported from the recording head through a backward ink route to the ink tank. By this circulation flow of the ink fluid, bubbles in the ink fluid in the forward and backward ink routes and even in the recording head may be returned to the ink tank and released into the atmospheric at the ink tank.
Generally, the forward and backward ink routes used for establishing the circulation flow of ink as described above are also used for supplying the ink fluid to the recording head responsive to the ink ejection from the recording head. In supplying ink fluid to the recording head when ejecting ink droplets from the recording head, it is conventional that ink fluid is supplied not by drive force developed by the pump but by using capillary action in the ink route to the recording head.
When the ink supply system between the ink tank and the recording head is used both for supplying ink fluid for ejecting ink droplets and for forming circulation flow of ink fluid, the ink supply system should satisfy the following condition, especially when the filter is installed in the ink supply system. When the circulation flow of ink fluid is formed, bubbles should return to the ink tank finally after passing through the filter, and when ink droplets are ejected in the recording operation, bubbles should not pass through the filter.
In the filter installed in the ink supply system in the prior art, however, it is often difficult to satisfy the condition described above.
FIGS. 1 and 2 are schematic cross-sectional views showing a part of an ink supply system of the prior art where the filter is installed.
In a dust removal unit of the prior art as shown in FIGS. 1 and 2, it is difficult to satisfy the above condition related to the filter. For example, in FIG. 1, ink fluid 2 supplied in the direction shown by an arrow "a" flows through an inlet tube 21 to a dust removal unit 20 having a filter 1. The ink fluid 2 passing through a fluid route 3 in the dust removal unit 20 is filtered in the filter 1 which blocks dust with a size greater than 10 .mu.m. Thus, dust 4 in the ink fluid 2 are captured at an upstream side of the filter 1 and do not flow into a downstream side of the filter 1 into an outlet tube 22.
When the circulation flow of the ink fluid is formed by a pump in the prior art ink supply system as described above as well as when ejecting ink droplets in a recording operation, not only dust and substances come out of ink but also bubbles can not pass through the filter 1 and stay at the upstream side of the filter 1. As a result, there may be a problem in which bubbles can not fully removed even by the ejection recovery operation.
In addition, these fine-sized bubbles collect and develop into large-sized bubbles, which stay at the upstream side of the filter 1. A large-sized bubble 5 may cause the following problem. Owing to the large-sized bubble 5, the effective filtering cross-section area of the filter 1 is reduced and the flow resistance of ink fluid passing through the dust removal unit 20 is increased, so that filtering ability is lowered. An example of the dust removal unit which solves this problem of the dust removal unit 20, is shown in FIG. 2. In FIG. 2, like parts as shown in FIG. 1 have like reference numerals. In this unit, as the overall cross-section area of the filter 1 is made to be large enough so that the filtering ability of the filter 1 is not reduced even when bubbles are captured within the filter 1. However, from experiments, it is known that the size of the bubble captured at the upper-stream side of the filter 1 even increases if bubbles are contained in the ink fluid 2 passing through the dust removal unit 20 having a filter 1 with larger cross-section area, and that the effective filtering cross-section area of the filter 1 can not increase in proportion as the overall cross-section area of the filter 1 increases.
In addition, the size of the dust removal unit shown in FIG. 2 becomes larger than the size of the dust removal unit shown in FIG. 1. This goes against the requirement for establishing the ink jet recording apparatus with a small size. Furthermore, a cost of materials used for the filter which can capture fine-sized particles is relatively high, and then, the larger cross-section of the filter 1 brings a disadvantage for cost reduction. Filters used in prior art dust removal units can not satisfy conditions both in the recording operation and the recovery operation with circulation ink flow, and the filter efficiency may decrease as the apparatus runs longer.